The present invention relates to a process and an apparatus for purifying a gas, particularly to a process and an apparatus for purifying a gas containing as major components oxygen and nitrogen and as impurities to be removed at least water, carbon dioxide and carbon monoxide. More specifically, the present invention relates to a process and an apparatus for purifying raw air so as to supply the treated air as clean dry air. The present invention also relates to a process and an apparatus for purifying a gas which are suitably realized in a pre-treating equipment for purifying a raw air to be supplied to a cryogenic separation unit by carrying out oxidation of trace hydrogen and trace carbon monoxide contained in the raw air to form water and carbon dioxide and remove them using adsorbents, respectively.
Recently, there is an increasing demand for high-purity gases, and particularly nitrogen gas, which is consumed in large amounts in electronics industries especially in the semiconductor manufacturing process, is required to be of highest possible purity. Nitrogen gas utilized in the field of electronics should be treated to remove chemically active impurities such as oxygen, water, carbon dioxide, hydrogen, carbon monoxide and hydrocarbons contained therein so as to be as pure as possible.
When nitrogen gas is to be separated from air, the cryogenic separation process is suitably employed in view of the scale of production and gas purity. However, all of such impurities as described above cannot be separated from nitrogen by distillation according to the cryogenic separation process, but water and carbon dioxide are in many cases removed by means of adsorption in the raw air supply system. It is possible to remove hydrogen by means of distillation, but hydrogen is contained in a very small amount in air, so that the distillation treatment involves not a small loss of nitrogen, and an extra distillation column must be installed because of vapor-liquid equilibrium relation of these gases. Further, carbon monoxide and nitrogen have approximately the same boiling points, so that it is difficult to separate carbon monoxide from nitrogen by means of distillation.
In view of such technical difficulties, there have been proposed some methods for removing hydrogen and carbon monoxide from raw air. Japanese Patent Publication No. Hei 5-65782 discloses a method in which hydrogen and carbon monoxide contained in raw air are reacted with oxygen also contained therein under the heating condition with the maximum temperature of 190.degree. C. to be converted into water and carbon dioxide respectively which are then removed using a so-called pre-treating equipment usually installed in cryogenic separation units. In this method, in order to provide an optimum temperature for carrying out the oxidation reactions of hydrogen and carbon monoxide, raw air heated to 80 to 150.degree. C. by compression is further heated through a heat exchanger and a heater and then introduced to a catalyst column to effect oxidation of hydrogen and carbon monoxide. While this method is successful in improving efficiency of the apparatus by utilization of the heat of compression, heat recovery by heat exchangers, etc., it inevitably leads to increase in the power consumed for heating, variable expense and fixed expense for apparatuses or heat exchanger, etc.
Meanwhile, Japanese Unexamined Patent Publication No. Hei 4-219111 discloses a method in which adsorption units employed as pre-treating equipments for the cryogenic separation unit, which removes water and carbon dioxide, are allowed to have a three-layer structure consisting of a desiccant, a catalyst and a CO.sub.2 adsorbent, and water, carbon dioxide, hydrogen and carbon monoxide contained in raw air are designed to be removed simultaneously. According to this method, hydrogen and carbon monoxide are also oxidized to be converted into water and carbon dioxide which are then removed using the adsorbent. This method enjoys merits in that it employs raw air having a temperature of 5 to 50.degree. C. to require no heating procedure and that it requires no extra catalyst column for carrying out oxidation of hydrogen and carbon monoxide, since the catalyst is packed into existing pre-treating adsorption units. However, to pack a catalyst between a desiccant and a CO.sub.2 adsorbent in each adsorption unit affects significantly regeneration of the adsorbent. More specifically, when thermal swing adsorption (TSA) process is employed, the catalyst is packed on the upstream side (with respect to the regenerated gas introducing path) of the desiccant which requires the greatest heat in this process, so that increase in the heat necessary for heating and in the amount of gas to be used for the regeneration cannot be avoided. Meanwhile, when pressure swing adsorption (PSA) is employed, there is no problem in the heating value to be consumed for regeneration, but it causes increase in the amount of purge gas due to increase in the capacity of the adsorption units or loss of raw air to be caused by the operation of switching between adsorption units.